Considerable effort has historically been spent in both broad and specific development of sensor technologies and it is still an area of vital importance and interest to the medical, manufacturing, environmental and defense/security communities.
The most successful sensors for determining gases are electrochemical sensors but their use is limited by the stability of the electrode surface and by instabilities in the gas diffusion barrier, because they usually measure the rate of diffusion of the gas to the cathode or to the anode. They are not welcome for in vivo investigations due to the probability of electroshock. For these reasons, photochemical sensors have been developed to determine gas concentrations. Many of them contain organic dyes or organo-metallic compounds as sensing compounds immobilized to gas permeable supports. Immobilization of the sensing compound onto the matrix of the support is a critical step in the fabrication of photochemical sensors since e.g. organic dyes interact directly with the surface of the matrix of the supports so that the properties of the sensing compound strongly depend on the properties of the supportive matrices.
The reproducibility of the chemical surface composition and the surface reactivity of the surface matrices have been proved to be crucial in sensor applications. This is particularly critical in the case of gas sensors where the surface reactions are the origin of the gas detection mechanism.
In 1990 Lang et al. used metal oxide membranes to develop a catalytic gas sensor (EP-0 421 158 A1). Further, U.S. Pat. No. 5,624,640 discloses a sensor for detecting nitrogen oxide having a semiconducting metal oxide layer (TiO2, ZrO2, SiO2 and/or Al2O3) with a platinum content ranging from 0.01 to 20 weight percent.
U.S. Pat. No. 5,490,490 discloses a gas sensor used for internal combustion engines where the sensor body consists of a porous high-temperature fluorescent inorganic oxide ceramic to generate an optical fluorescence signal.
U.S. Pat. No. 6,251,342 comprises an optical fiber where at least a part of the surface is coated with a sol-gel processed porous uniform mixture of matrix material (alumina, zirconia, titania or silica) or a mixture of any of them with ceramic fluorescent indicator like Cu-ZSM-5 zeolite incorporated therein.
U.S. Pat. No. 5,979,423 describes a fiber-optic gas composition sensor in which sensor body is made with a porous high-temperature fluorescent inorganic oxide ceramic.
Also, there are some relevant papers which use mesoporous semi-conducting oxides for sensor applications. Liu et al. (Anal. Chim. Acta 341, 161 (1997)) propose a potentiometric biosensor for urea based on the immobilization of urease on a gamma-Al2O3 substrate. Nakagawa et al. (Anal. Sci. 14, 209 (1998)) developed a chemiluminescence sensor made with Al2O3 for determining organic molecules in water, Zhang et al. (Mol. Cryst. Liquid Cryst. 337, 489 (1999)) describe microporous aluminum oxide membrane-based optical interferometric sensor for determining stearic acid and Lazarin et al. (J. Membrane Sci. 221, 175 (2003)) use a highly dispersed Al2O3 in a cellulose acetate membrane modified by attaching an organofunctional group to develop a platinum electrode for determining dissolved oxygen.
Other investigations in the gas sensor fields show that it is possible to use gas-sensitive solvent polymer membranes to which is added at least one gas-selective ionophore as well as a chromoionophore and additives such as plasticizers, counterions etc to develop gas-sensor devices (U.S. Pat. No. 6,704,470 B1, U.S. Pat. No. 5,494,640 and Nezel et al. (Chimia 55, 725 (2001)).
Since the present membrane gas sensors have relative low sensitivity values towards the corresponding gas there is a strong need to provide gas-selective compound charged membranes which show an enhanced sensitivity.